The eight largest nuclear plant operators in Europe were faced with €100 billion (£70bn) of debt for nuclear clean-ups last year.
The figure accounts for 22% of their total debt, according to financial services firm Standard & Poor’s report.
It adds European utilities are setting aside “large sums to cover the costs of decommissioning nuclear power plants and disposing of nuclear fuel waste in the next two decades”.
International and national regulations require nuclear operators to provision for these future costs.
However the report states the accelerated nuclear phase-out in Germany has sparked fears the provisions might not cover the costs.
There are concerns as to whether companies have the financial flexibility to cope with potential changes in the valuation of these provisions in the coming years, it claims.
The report adds: “Given the tight headroom in the credit ratings of European utilities with heavy nuclear exposures any changes in the valuation of their provisions could drag on their credit metrics and liquidity and ultimately their credit ratings.”
It states there are also other challenges including “the risk of further political regulations such as environmental requirements or faster phase-outs, the lack of valuation and accounting standardisation between countries and operational challenges of carrying out the dismantling process and nuclear waste management”.
A recent report found a breakthrough in nuclear waste storage could save millions of pounds.